The present invention relates to fluorescent compositions including a fluorescent dye and a resin carrier, and, more particularly, to novel fluorescent compositions including an organic fluorescent dye and polyamide resin which are useful as colorants or pigments.
U.S. Pat. Nos. 2,938,873, 3,198,741, 3,412,036, 3,812,054 and 3,922,232 disclose various fluorescent compositions made from thermoplastic resinous materials which incorporate dyes for use as coating compositions and pigments. Of particular interest herein is the use of condensation resins as carriers for fluorescent dyes to provide fluorescent pigments useful as colorants for plastics. These materials have not been found to display the heat stability and other properties at temperatures and conditions required by certain polymer processing firms.
U.S. Pat. No. 3,915,884, which is assigned to the assignee of this application, teaches low molecular weight modified polyamides of improved heat stability. However, even these materials have not been entirely satisfactory. For example, the use of such pigments in blow molding techniques has been characterized by plateout or the tendency to deposit organic material such as oligomeric species on mold surfaces. Heretofore, modifications of the polyamides of the '884 patent when used with specially selected additives were believed to have a lower degree of plateout in blow molding applications than other commercially available fluorescent pigment and were considered to set the standard in the industry.
In addition to heat stability and resistance to plateout, fluorescent pigments should be friable or grindable so that they may be reduced to relatively small particles for incorporation into plastics and other materials to be colored. For example, typical pigment particle size ranges were from 10 to 50 microns. As indicated by the patentee in the '884 patent, the ability to provide particles of small size was previously thought to be hindered by increasing molecular weight of the resin carrier. Presumably, it is for this reason that the '884 resin carriers are of relatively low molecular weight, e.g. less than 2,500.
The fluorescent dyes may be dissolved in the carrier resin to provide the pigment. Typical dye concentrations range up to about 15% based on the weight of the pigment.
For plastics processing, a masterbatch comprising pellets of a suitable plastic containing a relatively high concentration of pigment may be prepared by melt blending to provide a color concentrate for combination with the feed stock of plastics materials. The concentration of fluorescent composition or pigment in such prior art pellets has been limited usually to about 25 to 30% by weight due to the difficulty of forming uniform pellets which are free of fibrils at higher concentrations. The occurrence of fibrillation is believed to be associated with difficulties in solvating or dispersing the pigment at higher concentrations. It is desirable that relatively higher concentrations be achieved since this enables more efficient coloration.